Pentagonite is a rare secondary mineral typically found as stunning deep-blue radiating sprays or bow-tie clusters within volcanic vugs. It is chemically dimorphous with its much more common sister mineral, Cavansite, but displays a distinct orthorhombic crystal structure. Collectors prize it for its intense, vibrant color and aesthetic crystal habits, found almost exclusively in the Deccan Traps basalt quarries of India.
Is this pentagonite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch pentagonite with a known reference. Pentagonite sits at Mohs ~3-4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: bluish white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: greenish blue.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: prismatic, rosette, twinning common.
Often confused with
Pentagonite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside pentagonite
Minerals reported to co-occur with pentagonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca(V⁴⁺O)Si₄O₁₀·4H₂O
- Mohs hardness
- ~3-4
- Density
- 2.33 g/cm³
- Colors
- Streak
- Bluish white
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Prismatic, rosette, twinning common
- Cleavage
- Good on {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In coatings on a tu®
Where rockhounds find pentagonite
Classic worldwide localities
- Three km south of Owyhee Dam, Malheur Co., Oregon, USA
Field-hunting tip
Where it forms: In coatings on a tu®. If you start seeing cavansite, heulandite, stilbite in float, you are in the right ground. Typical habit: prismatic, rosette, twinning common. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify pentagonite?+
Where is pentagonite found?+
Is pentagonite safe to handle?+
What rocks look like pentagonite?+
What minerals are found with pentagonite?+
What kind of rock does pentagonite form in?+
What is pentagonite used for?+
Find pentagonite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
